Authors will describe characters explicitly either by observation of another character or via a narrator. The purpose of this is to allow the reader to get to know the character intimately in order to use any behaviour that they display later as understandable based on how they were described when they were introduced.
Building the characters also allows the writer to engage the reader's emotions by giving the character likable, unlikable or relatable traits. These are often full of descriptive words and very intimate details about the Character that sometimes only they themselves will know.
